Harvesting energy from a electromechanical energy conversion system energy converter using Dielectric Elastomer (DE), as sort of Electro Active Polymer (EAP), requests for special way of charging and discharging the DE. Such an electromechanical energy conversion system using an EAP based device is for example disclosed in WO2010/146457, embodied as a wave energy converter.
Such an EAP based device can be considered as a variable capacitor of which the capacitance changes as a function of the amount of deformation exerted on a layer of EAP material.
As so far, some types of converters are built and tested. Usually they are boost step-up (for charging) and buck step-down (for discharging) converters. Because of electrical and conversion losses that occur special attention is paid to current shape during charging and discharging.
For discharging of the variable capacitor of the EAP based device at the moment it is contracted, buck (generally step-down) converters are used. A main reason is that the power sink/source, for example a battery, has lower voltage than the variable capacitor in the contracted state of the EAP based device.
A way of choosing a right topology for buck conversion in discharging function comes from different conditions. It is desirable to have buck conversion with low voltage switches, easy control of current, small inductor, low switching frequency, small losses and, of course, to keep the topology simple as much as possible in terms of hardware structure and control.
From the aspect of energy harvesting it is important to keep losses relatively small. Conversion losses will be minimal if an optimal way of charging and discharging is used, and this includes control of the current shape during charging and discharging of the variable capacitor of the EAP based device. Electrical losses will occur if there is a lot of current ripple, but avoiding this means that converter should work in CCM (Continuous Conduction Mode) and then switching losses will be increased because a higher switching frequency must be achieved. New SiC semiconductors, for example, have lower switching losses, so they seems to be reasonable choice for use in step-up converters and step-down converters.
Additionally, high current amplitudes are expected to produce higher electric losses due to the high series resistance of the EAP material. On the other hand, allowing only relatively low current amplitudes adversely affect the conversion efficiency by increase of the charging time of the EAP based device before a maximum electric field in the EAP based device is reached, which leads to higher conversion losses.
It is an object of the present invention to provide a charging/discharging circuit for an electromechanical energy conversion system and an electromechanical energy conversion system that overcomes the disadvantages from the prior art.